1. Field of the Invention
The present invention relates to a fixing apparatus for fixing an image on a recording material by applying pressure, which is suitably used in image forming apparatuses such as copiers and printers employing electrophotographic and electrostatic recording systems.
2. Related Art
An image forming apparatus employing an electrophotographic system such as a copier, printer or facsimile forms an electrostatic latent image by charging the surface of an electrophotographic photosensitive body by means of a charging apparatus and exposing the surface of the charged electrophotographic photosensitive body by means of an exposing apparatus. The electrostatic latent image is developed by a developing apparatus to form a toner image which is transferred to a recording material such as paper by a transferring apparatus and then fixed on the recording material by a fixing apparatus as a permanent fixed image, and the fixed image is output.
FIG. 8 is a schematic diagram showing an image forming apparatus according to an embodiment of the present invention. The above-mentioned fixing apparatus operates as follows. A recording material P such as paper to which a toner image has been transferred by a transfer nip between a photosensitive drum 2 and a transfer roller 5 as an image carrier is conveyed while being held by a fixing nip formed between a heating roller 12 and a pressure roller 13 of a fixing apparatus 7 through a conveyance guide 9 and the unfixed toner image is thermally fixed on the recording material P as a permanently fixed image by heat from a heater (halogen heater) 14 incorporated in the heating roller 12 and pressure from the fixing nip. Other constituent elements of the image forming apparatus 1 will be described with reference to embodiments of the present invention which will be described below.
When an envelope is conveyed as a recording material in the image forming apparatus constituted as described above, if the envelope is nipped and conveyed while it is heated by the fixing apparatus, there arises a problem that it wrinkles. The envelope is formed by folding a single sheet of paper and pasting the overlapped portions. If there is a difference in shrinkage direction between the front side and the rear side of the sheet, there is no escaping for distortion at the time of shrinkage by heating, with the result that the sheet of paper is apt to wrinkle. To prevent wrinkling, a method making use of pressure means for reducing the contact pressure of the pressure roller is generally employed.
For example, FIG. 6 shows an example of a pressure mechanism for the fixing apparatus 7. The pressure mechanism comprises a heating roller 12 including a heater 14 (halogen heater), a pressure roller 13 contacting the heating roller 12, and a frame 52 supporting these components. Both ends of the heating roller 12 are rotatably supported by the frame 52 through the intermediation of a bearing (not shown). One end of the heating roller 12 is provided with a gear (not shown) to be driven by drive force transmitted from a drive source. Both ends of the pressure roller 13 are rotatably supported by a support member 15 through the intermediation of a bearing (not shown). Reference numeral 16 denotes a rotatable lever which is a contact pressure switching means and movably supported by a shaft 52a provided on either side of the frame 52. The rotatable lever 16 comprises an operation portion 16a, a contacting portion 16b contacting the support member 15 and a stopping portion 16c for stopping the movement of the rotatable lever 16. Reference numeral 60 denotes an elastic member made of a coil spring for biasing the support member 15 and provided between the frame 52 and the support member 15. An axis hole 15a which serves as a fulcrum is formed in the support member 15 so that the support member 15 is movably supported by a shaft 52b provided on either side of the frame 52. On the free end side of the support member 15 is formed a bending portion 15b which serves as a contacting potion with the above-mentioned rotatable lever 16 and the support portion of the above-mentioned elastic member 60.
When an envelope is to be conveyed, the rotatable lever 16 is first rotated clockwise as shown in FIG. 7. Since the contacting portion 16b of the rotatable lever 16 contacts the bending portion 15b of the support member 15, and the rotatable lever 16 is biased in the direction indicated by an arrow in FIG. 7 (center direction of the shaft 52a ) by the support member 15, the moment generated by the biasing force of the elastic member 60 does not act on the rotatable lever 16, and the rotatable lever 16 is stopped at a position shown in FIG. 7, and the support member 15 is kept down against the biasing force of the elastic member 60.
Since the rotatable lever 16 has a stopping or detent portion 16c, it is prevented from moving more than required. The support member 15 is forced down by the operation of the rotatable lever 16 with the fulcrum 15a as the center. The pressure roller 13 supported by the support member 15 is kept at a position at a predetermined distance from the heating roller 12.
The pressure roller 13 comprises a rubber layer 13b formed on the surface of a metal core 13a which is in contact with the heating roller 12 to obtain a nip width. Since rubber has a large thermal expansion coefficient and a low heat conductivity, the temperature of the pressure roller 13 gradually rises and the outer diameter of the pressure roller 13 gradually increases (broken line portion in FIG. 7) according to the operation time of the image forming apparatus 1.
Since the biasing force of the elastic member 60 is extremely large, even when the outer diameter of the pressure roller 13 becomes large, the rubber layer 13b is deformed and the pressure roller 13 cannot move down easily. Owing to this constitution, the contact pressure to the heating roller 12 of the pressure roller 13 or the nip width increases, thereby causing the problem of the wrinkling of the envelope.
When the amount by which the rotatable lever 16 is forced down (distance between the pressure roller 13 and the heating roller 12) is set taking into account the change in the outer diameter of the pressure roller 13 by thermal expansion to prevent wrinkling, the unfixed toner image will not be fully melt-fixed until the temperature of the pressure roller 13 is increased to a sufficient degree, a fixed outer diameter thereof is achieved and a predetermined pressure is obtained. In particular, when the thermal expansion of the pressure roller 13 is large, the heating roller 12 and the pressure roller 13 are set in a state that they do not contact each other, so that no heat is transmitted from the heating roller 12 to the pressure roller 13, making it necessary to provide a heater 14 in the pressure roller 13 as well.
That is, the wrinkling of the envelope and the melt-fixing of the unfixed toner image are contradictory and a long waiting time is required until the fixing apparatus 7 of the image forming apparatus 1 is fully warmed so as to prevent the wrinkling of the envelope and melt-fix the unfixed toner image.
It is an object of the present invention to provide a fixing apparatus capable of fixing a toner image on a recording material without wrinkling the recording material.
Another object of the present invention is to provide a fixing apparatus capable of fixing a toner image on an envelope.
Still another object of the present invention is to provide a fixing apparatus capable of changing fixing pressure.
Still another object of the present invention is to provide a fixing apparatus comprising:
a fixing rotator in contact with a recording material supporting an unfixed image;
a pressure rotator in pressure contact with the fixing rotator;
a support member supporting the pressure rotator and swingable around a plurality of fulcrums as its center;
an elastic member for biasing the support member so that the pressure rotator is biased by the fixing rotator; and
pressure contact force changing means for changing pressure force by swinging the support member against the biasing force of the elastic member, wherein
the pressure contact force changing means can change pressure contact force while the pressure rotator is in pressure contact with the fixing rotator.
Other objects of the present invention will become apparent from the following description.